1. Field of the Invention
The present invention relates to solar cells and particularly to an improvement in solder coating of electrodes included in the same.
2. Description of the Related Art
An exemplary solar cell including electrodes conventionally coated with solder is schematically shown in cross section in FIG. 1. In the figures of the present application, like portions are denoted by like reference numerals.
The FIG. 1 solar cell includes an etched p-type silicon substrate 1 having a light receiving side with an n-type diffusion layer 2. On n-type diffusion layer 2 an anti-reflection film 3 is provided to reduce surface reflectance. P-type silicon substrate 1 has a back surface provided with a back surface aluminum electrode 4. Back surface aluminum electrode 4 and anti-reflection film 3 on the light receiving side are provided thereon with silver electrodes 5 and 6 coated with solder layers 7.
Such a solar cell is fabricated by such a method as represented in a flow chart of FIG. 2. More specifically, in a case of using a crystalline silicon substrate, p-type silicon substrate 1 is initially etched at step S1. At step S2, p-type silicon substrate 1 is provided on its light receiving side with n-type diffusion layer 2 and thereon is provided anti-reflection film 3 to reduce surface reflectance.
At step S3, p-type silicon substrate 1 has its back surface almost entirely screen-printed with aluminum paste. The printed aluminum paste is dried and fired in an oxidizing atmosphere to form back surface aluminum electrode 4.
At steps S4 and S5, silver paste is screen-printed on back surface aluminum electrode 4 and anti-reflection film 3 in patterns and then dried. The dried silver paste is fired in an oxidizing atmosphere to form silver electrodes 5 and 6. That is, silver electrodes 5 and 6 can be formed by simultaneous baking (step S6).
At step S7, substrate 1 is immersed in an activator-containing flux at a normal temperature for several tens seconds to provide silver electrodes 5 and 6 with the flux. Then, substrate 1 is exposed to hot air and thus dried.
At step S8, substrate 1 is immersed in a 6:4 eutectic solder bath (of about 195xc2x0 C.) containing 2 mass % silver for about one minute to coat silver electrodes 5, 6 with solder layers 7.
At step S9, substrate 1 is ultrasonically washed several times in normal or hot water and it is then rinsed with pure water and finally exposed to hot air and thus dried. A conventional solar cell is thus obtained.
FIG. 3 shows a solar cell string including a plurality of conventional solar cells thus fabricated and interconnected. In this conventional string, a solar cell 10 has a main surface electrode 11 coated with 6:4 solder and a plurality of such solar cells 10 are connected by interconnectors 12 coated with 6:4 solder. Such a string is fabricated in such a method as follows. Interconnector 12 including a copper core line coated with 6:4 eutectic solder is superposed on main electrode 11 of solar cell 10 and exposed to blowing hot air of about 400xc2x0 C. to melt the solder. The solder is then cooled and thus solidifies to provide the connection. Such a connection process is repeated for the plurality of solar cells on their front and back sides to provide a cell string. The string thus completed is used to fabricate a solar cell module.
In recent years, lead harmful to human body causes issues from an environmental view point and thus various electronic devices free of lead are increasingly developed. Fabrication of solar cells free of lead is also demanded in the industry of interest.
In the past, however, a solar cell using lead-free solder has not been produced. For example, if a conventional 6:4 eutectic solder bath is replaced with a Sn bath to coat with Sn an electrode formed of fired silver paste, the silver contained in the electrode would be taken into the Sn bath and the electrode would disappear in some locations and the product would not function as a solar cell. This is probably attributed to the fact that Sn has a melting point of 231.9xc2x0 C., about 50xc2x0 C. higher than that (i.e., 183xc2x0 C.) of 6:4 eutectic solder.
U.S. Pat. No. 5,320,272 discloses an example of lead-free solder, which, however, is used for semiconductor integrated circuits.
In view of the above-described prior art, an object of the present invention is to provide a solar cell having good output properties without causing lead pollution. Another object of the present invention is to provide an interconnector which does not cause lead pollution and then provide a reliable solar cell string connected by such interconnectors.
A solar cell according to the present invention is characterized in that it has electrodes coated with lead-free solder.
The electrode itself can be formed by baking matal paste. Furthermore, the electrode may be formed by metal vapor deposition, spattering, or plating.
The lead-free solder can preferably be Snxe2x80x94Bixe2x80x94Ag-based solder or Snxe2x80x94Ag-based solder.
The electrode is preferably formed from matal paste containing powdery silver, powdery glass, an organic vehicle, an organic solvent, phosphorus oxide, and halide.
The solar cell""s electrode receives flux including resin, a solvent, and a resin stabilizer, before it is coated with lead-free solder.
An interconnector for the solar cell according to the present invention is characterized in that it is coated with lead-free solder.
A solar cell string according to the present invention is characterized in that a plurality of the solar cells having the electrodes coated with the lead-free solder are interconnected by the interconnectors coated with the lead-free solder.
The lead-free solder used for the solar cell and that used for the interconnector can be identical in composition.
At least one of the lead-free solder for the solar cell and that for the interconnector can contain Bi preferably at 3 to 89 mass %.
At least one of the lead-free solder for the solar cell and that for the interconnector may contain Ag preferably at 3.5 to 4.5 mass %.
In the solar cell according to the present invention, the electrodes can be protected from mechanical shock and moisture in the ambient by coating the electrodes with the lead-free solder which does not cause lead pollution. Coating the electrodes with the lead-free solder facilitates formation of the solar cell string by interconnecting the plurality of the solar cells with the interconnectors.
FIG. 4 is a schematic cross section of an example of a solar cell according to the present invention. The FIG. 4 solar cell is different from the FIG. 1 conventional solar cell only in that electrodes are coated with different solder layers. More specifically, the present solar cell uses lead-free solder layers 8, rather than conventional 6:4 eutectic solder layers.
The lead-free solder can be Snxe2x80x94Bixe2x80x94Ag-based solder or Snxe2x80x94Ag-based solder. Snxe2x80x94Bixe2x80x94Ag-based solder and Snxe2x80x94Ag-based solder each have a melting point lower than Sn. Herein, Snxe2x80x94Bixe2x80x94Ag-based solder contains no less than 0.1 mass % Ag. Snxe2x80x94Ag-based solder also contains no less than 0.1 mass % Ag.
To carry out the solder dip process without causing problems, it is desirable to use a conventional dip temperature of about 195xc2x0 C. and it is necessary that the dip temperature is no more than 225xc2x0 C. which is a practical limit in view of solar cell characteristics and reliability. To have a melting point of no more than 225xc2x0 C., Snxe2x80x94Bixe2x80x94Ag-based solder containing 0.1 mass % Ag should contain 5 to 88 mass % Bi and that containing 1.3 mass % Ag should contain 3 to 89 mass % Bi. To have a melting point of no more than 195xc2x0 C., Snxe2x80x94Bixe2x80x94Ag-based solder containing 0.1 mass % Ag should contain 27 to 79 mass % Bi and that containing 1.8 mass % Ag should contain 35 to 60 mass % Bi. Thus, Snxe2x80x94Bixe2x80x94Ag-based solder containing 3 to 89 mass % Bi is preferable and that containing 35 to 60 mass % Bi is more preferable.
Similarly, Snxe2x80x94Ag-based solder having a melting point of no more than 225xc2x0 C. should contain 3.5 to 4.5 mass % Ag. However, there does not exist Snxe2x80x94Ag-based solder having a melting point of no more than 195xc2x0 C. Thus, Snxe2x80x94Ag-based solder containing 3.5 to 4.5 mass % Ag is preferable.
The solar cell""s electrode can be formed from silver paste containing powdery silver, powdery glass, an organic vehicle and an organic solvent as main components and also containing iridium chloride and phosphorous oxide. Furthermore, to coat the solar cell electrode with lead-free solder, a flux material can be used which contains a polyalkylglycol-type resin and a solvent and does not contain any activator. More specifically, a flux containing a resin, a solvent and a resin stabilizer can be used to clean the electrode before lead-free solder is used to coat the electrode.
The solar cell electrode can be formed of matal paste fired and it can alternatively be formed by metal vapor deposition, spattering or plating.
The present interconnector for the solar cell is coated with lead-free solder.
In the present solar cell string, a plurality of solar cells each having electrodes coated with lead-free solder are interconnected by interconnectors each including a metal core line coated with lead-free solder.
FIG. 5 schematically shows a solar cell string of the present invention. In this string, a solar cell 10 has a main surface electrode 21 coated with lead-free solder and a plurality of such solar cells 10 are connected by interconnectors 22 each including a metal core line coated with lead-free solder. Herein, if the lead-free solder used for the solar cell and that used for the interconnector are identical in composition, the product can be fabricated in a simplified and stabilized process. If the lead-free solder for the solar cell and that for the interconnector are different in composition, they have their respective different melting points and thus the soldering process requires precise temperature adjustment.
By introducing Ag into at least one of the lead-free solder for the solar cell and that for the interconnector, the following effect can be obtained. By introducing Ag into the lead-free solder for the solar cell, it becomes possible to significantly delay elusion of Ag contained in the solar cell electrode of Ag paste fired. On the other hand, even if the solder for the solar cell electrode does not contain Ag, it becomes possible by introducing Ag into the lead-free solder for the interconnector to reduce elusion of Ag from the electrode of fired Ag paste in the soldering process.
Furthermore, if at least one of the lead-free solder for the solar cell and that for the interconnector contains Bi, the solar cell electrode and the interconnector""s metal core line can be connected together without reducing the melting point of the other lead-free solder. Herein, at least one of the lead-free solder preferably contains 3 to 89 mass % Bi, more preferably 35 to 60 mass % Bi. Similarly, at least one of the lead-free solder for the solder cell and that for the interconnector is Snxe2x80x94Ag-based solder, it preferably contains 3.5 to 4.5 mass % Ag.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.